Conventionally, in an imaging device such as a digital camera, with advance in a high performance of a lens barrel having a zoom lens function capable of changing a focal distance, or miniaturization of the imaging device according to the user's demand, there has been increasingly used a collapsible lens barrel in which a plurality of lens groups are collapsed when is not in photograph.
Furthermore, because of the demand for not only simple reduction in dimension, but also further reduction in thickness, it is now important to reduce the thickness of the lens barrel in a collapsed position to the maximum position. As a technology to cope with the demand for reduction in thickness of the lens barrel, there has been used a collapsible lens barrel in which a lens cylinder retaining therein a plurality of lens groups is collapsed into the imaging device when not in use, and one of the lens groups is retracted out of an optical axis of the lens groups when the lens groups are collapsed.
A technology for retracting one of lens groups out of an optical axis is disclosed in, for example, JP No. 2003-315861 A (Patent Document 1) and JP No. 2003-149723 A (Patent Document 2). According to the structures disclosed in these Patent Documents 1 and 2, since one of a plurality of lens groups disposed on the optical axis is retracted out of the optical axis when the lens groups are collapsed, the entire dimension of the lens barrel in a direction of the optical axis can be reduced.
However, in the structures disclosed in Patent Documents 1 and 2 as described above, the lens retracted out of the optical axis is substantially positioned within a collapsible lens cylinder provided in the lens barrel to retain therein the lens groups, even after the lens is retracted. Therefore, a space sufficient to contain the retracted lens must be provided in the lens cylinder. Providing such a space in the lens cylinder results in a larger diameter of the lens cylinder, eventually a larger size of the lens barrel within a plane transverse to the optical axis.
Accordingly, there is a problem that the conventional lens barrel has a large diameter by provision of the space containing the retracted lens in the lens barrel.
Meanwhile, substituting for a conventional silver-salt camera using a silver-salt film, there has been used rapidly a camera of a type as referred to as a digital camera or electronic camera and generalized in which a subject is imaged on a solid-state image sensing device such as a CCD (charge-coupled device), for example, to acquire imaging data of a still or moving subject, and the imaging data are recorded digitally in a non-volatile semi-conductor memory or the like which is typified by a flush memory.
Digital cameras have a very large market, and needs of users for the digital cameras include various things. High-quality picture and miniaturized digital cameras are desired always by the users and occupy a large weight.
Therefore, both high performance and miniaturization are required for a zoom lens used as a photographic lens. In a miniaturized camera, it is necessary to reduce the entire length of a lens unit when it is used, that is to say, a distance between the nearest lens surface of the lens unit to a subject and an imaging plane of the camera.
It is also important to reduce the entire length of the lens unit when it is collapsed in a body of the camera by thinning each lens group of the lens unit. Furthermore, it is required for the high performance of the zoom lens that the zoom lens has a resolving power which corresponds to the image sensing device having at least about three to five million pixels or more throughout a zoom area.
A many of users desire a photograph lens having a wide angle, it is desired that a semi-field angle at a wide angle position of a zoom lens is 38 degrees or more. The semi-field angle of 38 degrees corresponds to a focal length of 28 mm in the conversion to a silver salt camera of 35 mm using a silver salt film of 35 mm size or Leica size. Furthermore, a camera having a possible large zooming changing magnification ratio is also desired. If a zoom lens has a focal length of a range of 28 to 135 mm in the conversion to the silver salt camera of 35 mm, the zoom lens can take usual photographs almost. A changing magnification ratio of such a zoom lens is about 4.8 times.
Therefore, it is needed that a similar to or more changing magnification ratio than the above-mentioned changing magnification ratio is requested for a digital camera.
Although various zoom lens units for digital cameras are used, it is difficult to decrease the entire thickness of a lens unit including five lens groups or more, the lens unit is not suitable for miniaturization of the lens unit.
There is known a zoom lens unit having a changing magnification ratio of three (3) times, as a more general type. The zoom lens unit includes a first lens group having a negative focal length or negative refracting power, a second lens group having a positive focal length or positive refracting power, and a third lens group having a positive refracting power which are disposed in sequence from a subject, and an aperture stop disposed in a side of the second lens group facing the subject and moving with the second lens group. According to a changing magnification operation from an end of a short focal length to an end of a long focal length, the second lens group is monotonously moved from an imaging plane to the subject, and the first lens group is moved to correct variations in the position of the imaging plane due to the changing magnification operation.
However, such a zoom lens unit is not suitable for a high changing magnification ratio structure having four (4) times or more.
For example, in JP H 11-174330 A and so on, there is disclosed a zoom lens unit including a first lens group having a positive refracting power, a second lens group having a negative refracting power, an aperture stop, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power, which are disposed in sequence from a subject. The first and third lens groups are monotonously moved to the subject, the second lens group is held in a fixed position, and the fourth lens group is moved suitably, according to a changing magnification operation from a wide angle position to a telephoto position.
Moreover, for example, in JP H4-296809 A (JP. Pat. 3008380) and so on, there is disclosed a zoom lens unit in which first and third lens groups are monotonously moved to the subject, a second lens group is monotonously moved to an imaging plane, and a fourth lens group is moved suitably, according to a changing magnification operation from the wide angle position to the telephoto position.
Furthermore, for example, in JP 2001-56436 A and so on, there is disclosed a zoom lens unit in which a first lens group is held in a fixed position, a second lens group is monotonously moved to an imaging plane, and a third lens group is moved to a subject, according to a changing magnification operation from a wide angle position to the a telephoto position.
In other words, for example, as disclosed in JP S 62-24213 A and JP H 3-33710 A as a type suitable to a high changing magnification there is well known a zoom lens unit including a first lens group having a positive refracting power (focal length), a second lens group having a negative refracting power, a third lens group having a positive refracting power, a fourth lens group having a positive refracting power, which are disposed in sequence from a subject, and an aperture stop disposed adjacent the third lens group.
According to a changing magnification operation from a wide angle position to a telephoto position, the first and third lens groups are held in a fixed position, the second lens group is monotonously moved from the subject to an imaging plane, and the fourth lens group is moved to correct variations in the position of the imaging plane due to the changing magnification operation. The zoom lens unit of this type is used in many video cameras or one portion of digital cameras because small numbers of, two lens groups are moved and hence a structure of a lens barrel is simplified.
However, in such a zoom lens unit, because it is necessary to secure a great moved amount of the second lens group achieving the changing magnification operation almost, the aperture stop disposed adjacent the third lens group is disposed always separately from the first lens group. Therefore, there is a problem that the first lens group becomes a very large size when a wide angle is provided.
JP 2001-56436 A also discloses that an aperture stop is disposed between the second and third lens groups, the second and third lens groups are moved in the reversed direction with respect to each other, and a moved amount of the second lens group is decreased by taking the third lens group on a role of the changing magnification operation so that a small sized first lens group is provided. A similar structure is disclosed in JP H6-94997 A.
However, even in the zoom lens unit of this type, the first lens group is held in the fixed position, thereby because the entire length of the lens unit is generally constant, if the semi-field angle at the wide angle position increases to 38 degrees or more, a large-sized first lens group is formed. Accordingly, it is difficult to increase the semi-field angle at the wide angle position to 38 degrees or more.
As described above, it is not possible to increase the semi-field angle at the wide angle position to 38 degrees or more, in the zoom lens unit in which the position of the first lens group is fixed, the first lens group is preferably moved. A large size of the first lens group is prevented by setting the entire length of the lens unit in the wide angle shorter than that in the telephoto and a sufficient wide angle can be accomplished.
JP H11-174330 A and JP H4-296809 A disclose that the first lens group is monotonously moved to the subject, according to the changing magnification operation from the wide angle position to the telephoto position, as described above.
However, in the zoom lens unit disclosed in these patent documents, the semi-field angle at the wide angle position is limited to about 25 to 32 degrees, the wide angle is still insufficient.
Meanwhile, devices having a function to image a subject are being generalized from conventional still cameras, electronic still cameras, digital cameras or video cameras having a function to image a driving subject, to various information devices such as mobile information terminal devices. A zoom lens unit is generalized as the lenses used in the devices, a wider area of a high changing magnification ratio and a high performance are requested increasingly.
In particular, in a zoom lens for imaging a subject on an image-sensing device, it is required that the zoom lens has a resolving power which corresponds to the image-sensing device having at least about three to five million pixels or more throughout a zooming area, as described above. However, a further high aberration correction is requested for the zoom lens because a size of the image-sensing device is further reduced and an image-sensing device having a diagonal size of 6 to 9 mm is being practiced and a pixel pitch is 3 mm or less in the case of realizing three to five million pixels in the compact image-sensing device.
For example, if the pixel pitch is set to 2.5 μm, because a Nyquist frequency is two hundred numbers per mm and a diffraction limitation is also of a problem, an allowable amount of aberration correction is lesser than as in the silver-salt camera.
There is a strong demand to have a property of wide field angle to a photographic lens, it is preferable that the semi-field angle at the wide angle position of the zoom lens is at least 35 degrees, 38 degrees or more, if possible. As described above, the semi-field angle of 38 degrees corresponds to the focal length of 28 mm in the conversion to the silver salt camera of 35 mm using the silver salt film of 35 mm size or Leica size. In realizing such a wide field angle, off-axis aberration such as distortion aberration, chromatic aberration of magnification or the like is easy to often generate, hence a lens design is very difficult in cooperation with the fact that the pixel pitch of the image-sensing device is less.
With respect to the changing magnification ratio, if the zoom lens has a focal length of a range of about 28 to 135 mm (about 4.8 times) in the conversion to the silver-salt camera of 35 mm, almost normal photographs can be accomplished through the zoom lens.
As other type zoom lens suitable for a high changing magnification, there is known a zoom lens including a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, which are disposed in sequence from a subject, and an aperture stop disposed adjacent a side of the third lens group facing the subject, as disclosed in JP H11-109236 A, JP H 11-142733 A and JP H 11-242157 A. In a changing magnification operation from a wide angle position to a telephoto position, each lens group is moved or fixed so that an interval between the first and third lens groups is large, and an interval between the second and third lens groups is less.
JP S 62-24213 A, JP H 3-33710 and JP H 6-94997 A disclose that a fourth lens group having a positive refracting power is disposed in an imaging side of the third lend group in addition to the above-mentioned structure.
The conventional zoom lenses as described above have a changing magnification ratio of 5 times or more in either three lens group structure or four lens group structure, but they have no semi-field angle of 35 degree or more at the wide angle position.
Even in JP H 11-142733 A, in which an example of the most wide field angle is disclosed, the changing magnification ratio is a degree of 3 to 5 times and the semi-field angle is a degree of 25 to 34 degrees, the changing magnification ratio is limited to three (3) times in an embodiment disclosing the most wide semi-field angle of 34 degrees. Therefore, the conventional zoom lenses cannot respond to the recent request for the higher performance satisfying both the wider field angle and the high changing magnification ratio.